Method of making image orthicon pickup tube with high storage capacity



o 23, 1969. W. cm ET AL 3,474,511

METHOD OF MAKING IMAGE ORTHICON PICKUP TUBE WITH HIGH STORAGE CAPACITY Filed March 16, 1967 Fig.1

1,1 W' Al W14! 1 Fig.2

Inventors.-

Werner Or! Kurt Frank b /ILMIJQQ Attorney United States Patent US. Cl. 29-2514 Claims ABSTRACT OF THE DISCLOSURE A method and construction to increase the signal-tonoise ratio in image orthicon pickup tubes, by increasing the capacitance between the target and the screen electrode. Insulating material is evaporated, through a mask, upon the screen electrode'These insulating deposits are of a predetermined thickness, and provide a definite spacing between the screen electrode and the target, and there by the desired capacitance.

BACKGROUND OF THE INVENTION In image orthicon pickup tubes, the signal-to-noise ratio is proportional to the square root of the magnitude of the charge stored in a target. This charge, and accordingly the signal-to-noise ratio, may be increased through either applying a higher potential difference between the target and the screen electrode, or by increasing the capacitance between the target and the screen. Since the picture quality becomes degraded when higher potential difference are applied, it is desired to increase the target capacitance. This may be accomplished by increasing either the diameter of the target, or reducing the distance or space between the target and the screen.

An increase in the diameter of the target is undesirable because difficulties are encountered thereby, in manufacture. This is due to the condition that microphonic interference resulting from oscillation of the screen, becomes increased. Furthermore, increasing the target diameter results in a corresponding increase in the size of the tube. The latter requires larger dimensions of the camera and an increase in the required deflection energy, both of which are undesirable. Although the capacitance may also be increased by reducing the distance between the target and the screen, the microphonic interference is thereby, correspondingly, increased.

Attempts to suppress microphonic interference have been made in the past. One such method resides in the manufacture of television pickup tubes, in which the screen electrode is supported by elements projecting from the surface of the target. The height of the projections are of the order of magnitude of the distance between the scanning lines. In this method, the elements projecting from the surface of the target electrode may consist of grains of pulversized conductive or insulating material. In practice, however, it is difiicult, if not impossible, to carry out the distribution of the powder over the target electrode, in the proper manner. This arises from the condition that it is difiicult to prepare grains of uniform size and to secure them to the surface of the target electrode. This method has also the disadvantage that the supporting members may, in some cases, become visible in the television picture. Another method, attempted in the past, consisted of providing the screen electrode with a coherent insulating layer secured to one of its sides. This insulating layer served to support the screen with respect to the target electrode.

This latter method, however, has a serious disadvantage, discovered after the coherent insulating layer was evaporated onto the screen. Due, for example, to the different coefiicients of thermal expansion of the material of the screen electrode and of the insulating material, it is possible that the screen becomes distorted from its desired planar or flat form. At the same time, it is also possible that, as a result of such difference in the coeflicients of thermal expansion, the insulating material becomes partially separated from the screen. This condition results in spots on the surface of the photocathode or on the target electrode.

Accordingly, it is an object of the present invention to increase substantially the signal-to-noise ratio by increasing the capacitance between the target and the screen electrode. It is an object that this be achieved by providing that the space between the screen electrode and the target be substantially smaller than the distance between the scanning lines. The method, according to the present invention, is to avoid the disadvantages described above in connection with the method employed in the past.

In accordance with the present invention, a manufacturing method is provided for a target electrode assembly, in image orthicon pickup tubes, wherein the target electrode is made of semi-conductive material. The screen electrode is closely and adjacently located to the target electrode and spaced therefrom by supporting elements of insulating material. The supporting elements are distributed between the screen and the target electrodes, within the picture area. The supporting elements are formed by the process of evaporating insulating material, through a mask, onto the screen electrode. The evaporation is performed on that side of the screen electrode which, upon final assembly, faces the target electrode.

The mask through which the insulating material is evaporated upon the screen electrode, may itself be made of a screen or grid. During the evaporating process, this grid mask is placed in actual contact with the screen electrode. The source of the evaporated insulating material is located, during the evaporation process, at a distance from the screens, which is large compared to the size of their mesh. At the same time, the area covered by the source, is small compared to the distance between the source and the area of deposition.

It may be of advantage that the mask be comprised of a grid with smaller openings or mesh than those prevailing in the screen electrode upon which the insulating material is to be evaporated. In this manner, wider separation between the insulating supporting elements is realized. The manufacturing method may be similar to that employed in the conventional production of screen electrodes for pickup tubes, wherein a final process of electroplating is applied. In such manufacture, the electroplating process may be maintained until the size of the apertures or mesh is reduced to the desired amount. In carrying out the process, according to the present invention, it may also be advantageous to use a mesh size and orientation of the masking member, whereby the projections of the insulating material deposited on the screen electrode, do not form interference patterns (moire patterns) with the screen body.

An image orthicon pickup tube, as constructed with the design criteria of the present invention, distinguishes from the tubes in the known art, by its features of low microphony and high storage capacitance. Furthermore, the characteristic curve in the design of the invention, relating intensity of illumination to signal output, is gradually curved. This curvature inhibits sudden limiting of the signal amplitude when driven beyond saturation, as in the conventional tubes.

The preceding design of the present invention has a particular advantage from the viewpoint of the quality of the picture. Thus, the signal-to-noise ratio measured in a 3-inch tube, designed in accordance with the present invention, was found to be substantially higher than that for conventional tubes. In fact, the signal-to-noise ratio of this 3-inch tube exceeded even that of a 4 /2-inch tube, of conventional design. The design and construction as outlined in the present invention is also applicable to tubes with larger target electrodes such as prevailing in 4 /z-inch tubes or larger. This is due to the condition that the difiiculties arising from increased microphony occurring in such tubes, is likewise prevented by supporting the screen electrode as disclosed by the present invention.

Tubes designed on the basis of the present invention are particularly useful for picking up images having very dim objects. This is particularly applicable to astronomy, for example. Thus, due to the high capacitance of the picture elements, fading of the image because of the transverse conductance of the target, remains small. In conventional tube design, the supporting elements project from the surface of the target electrode, and are thus visible in the picture. Due to the method of the present invention, however, these supporting elements do not appear in the picture, because they are covered by the grid lines of the mesh. The insulating deposits applied with the process of the invention, may be a few microns thick, without the insulating material becoming detached. The screen and target electrodes are held together through electrostatic forces. Through the present invention, in which the insulation is discontinuous, the disadvantages prevailing with conventional tubes, are avoided. Thus, the adhesion of the screen electrode to the target electrode, is assured through electrostatic forces.

In the event that dust particles are present, or the glass of the target electrode contains minute grains or bubbles, spots or defects may appear within a picture area. Such spots or defects result from an increased deviation of the local capacitance of the target, due to a coincidence of the supporting elements of the screen with the surface peaks upon the glass target. In such cases, it may be of advantage to evaporate the material for the supporting elements upon the target electrode. The evaporation may be performed through a mask which has only a few perforations per square centimeter, and the diameters of the perforations are smaller than a picture element. With this construction, coincidence of the supporting elements with the peaks or elevations upon the surface of the glass target, is avoided, and accordingly the spots will not be visibleor if they are, they will be visible only to a minute degree. The evaporation can be accomplished through a net foil or mask where a relatively large distance prevails between the openings. For example, two millimeters or more may prevail between the apertures or perforations having a diameter of 50 microns. The apertures or perforations may be distributed about to mask in an arbitrary manner. This type of distribution and construction of the mask may be realized through a commonly-known method based upon irradiating a glass foil with nuclear particles. This process is followed by an etching of the holes at the spots where corrosion is initiated by the particles.

SUMMARY OF THE INVENTION A method of constructing a target electrode assembly, in image orthicon pickup tubes, comprising the steps of preparing a planar conductive screen having a grid pattern; mounting said screen upon a first support member having an aperture; placing a masking member over said screen; evaporating through said masking member insulating material upon said screen to a predetermined thickness; preparing a planar-semi-conductive target electrode; mounting said target electrode upon a second support member having a second aperture aligned with said first aperture so that said screen extends across said aperture; and securing said first support member to said second support member so that said insulating material evaporated upon said screen is disposed between said screen and said target electrode.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an enlarged plan view of a portion of the screen electrode, in image orthicon pickup tubes, and shows the layout of the discrete insulating projections or members, in accordance with the present invention;

FIG. 2 is a cross-sectional view taken along line AA in FIG. 1, and shows the construction whereby the deposited insulating members upon the screen electrode, maintain separation between that electrode and the target; and

FIG. 3 is an enlarged sectional view, and shows the constructional arrangement for supporting the screen and target electrode, in the proper relationship to each other.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, and in particular to FIG. 1, the reference numeral 1 designates an enlarged view of a portion of a screen electrode as used in an imageorthicon pick-up tube to produce the accelerating field for the secondary electrons emitted from the semi-conductive target electrode, as a result of the incidence of photoelectrons thereon. Such screen electrodes are produced by electroplating screens having openings of the order of 25 and conductor widths of the order of 6p.- Insulating projections are deposited at separate positions on the screen through the evaporation of insulating materials, upon the screen, by way of predesigned masks.

Barium fluoride (BaF has been successfully employed as the evaporated insulating medium. To simplify the manner in which the insulating projections are produced, a grid, of the same structure as the screen upon which the projections are to be evaporated, may be used as the mask. The insulating deposits, shown by the shaded areas in FIG. 1, may be produced with the grid, in rectangular form, laid directly upon the screen electrode on which the projections are to be formed. In the configuration of FIG. 1, the grid lines were oriented at an angle of 45 with respect to the screen lines of the electrode. The evaporation process may be carried out through the use of an evaporator of small dimensions, e.g., 1 cm., located at a distance of approximately 15 cm. from the screen. It is essential that a high vacuum be maintained during the evaporation process in order to maintain at a minimum, the scattering of the evaporated molecules and the deposits upon the rear of the screen, resulting therefrom.

FIG. 2 shows a cross-sectional view, taken along line AA in FIG. 1, of the screen electrode 1, the storage target 3 made of conductive glass, and the insulating support members 2. The thickness of the insulating layer is preferably within the range of 25y.

FIG. 3 is a cross-sectional view showing the construction of the target assembly. Although the figure is an enlarged view, it is not quite as enlarged as FIGS. 1 and 2. In FIG. 3, the screen electrode is again designated by the reference numeral 1, and the target made of conductive glass foil is designated by the reference numeral 3. The screen electrode is secured to an annular ring 4, whereas the glass foil is stretched upon a frame 5. The diameter and cross-section of the ring 4 are designed so that the screen as well as the inner edge surfaces of the ring lie directly upon the glass foil. The insulating spaces or gaps, necessary to provide the desired capacitance of the storage target, is maintained through the presence of the insulating support members 2, described in relation to FIGS. 1 and 2. Due to the small size of the support members, they have been omitted from the diagram of FIG. 3, for the sake of clarity-they would not contribute toward the understanding of the figure.

Various modifications may be applied to the invention. Thus, a variety of suitable apertured masks may be used, and the insulating projections may be made of varying size, and thickness of material. In particular, the form of the signal characteristic of the tube may be modified in a prescribed manner by varying the thickness of the evaporated insulating layer.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of image orthicon pick-up tubes difiering from the types described above.

While the invention has been illustrated and described as embodied in image orthicon pickup tubes, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any Way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended Within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. A method of construction of a target electrode as sembly, in image orthicon pickup tubes, comprising the steps of preparing a planar conductive screen having a grid pattern; mounting said screen upon a first support member having an aperture; placing a masking member over said screen; evaporating through said masking member insulating material upon said screen to a predetermined thickness; preparing a planar semi-conductive target electrode; mounting said target electrode upon a second support member having a second aperture aligned with said first aperture so that said screen extends across said aperture; and securing said first support member to said second support member so that said insulating material evaporated upon said screen is disposed between said screen and said target electrode.

2. The method as defined in claim 1, wherein said masking member is a grid member.

3. The method as defined in claim 1, wherein said masking member has a grid pattern identical with said grid pattern of said planar conductive screen.

4. The method as defined in claim 2, wherein the grid lines of said masking member are angularly disposed in relation to the grid lines of said planar conductive screen.

5. The method as defined in claim 4, wherein the grid lines of said masking member are disposed at an angle of degrees with respect to the grid lines of said planar conductive screen.

6. The method as defined in claim 1, wherein said insulating material is barium fluoride.

7. The method as defined in claim 1, wherein said predetermined thickness is within the range of 2 to 5 microns.

8. The method as defined in claim 1, wherein said masking member is in direct contact With said screen during the step of evaporating insulating material thereon.

9. The method as defined in claim 4, wherein the grid openings of said masking member are substantially larger than the grid openings of said target electrode.

10. The method as defined in claim 5, wherein the apertures of said masking member are distributed in a random manner and have a diameter smaller than a picture element.

References Cited UNITED STATES PATENTS 2,874,449 2/1959 De Rooy et al 29-2514 2,878,549 3/ 1959 Willner 2925.14 3,196,515 7/1965 Van Asselt et al. 2925.18 3,230,601 1/1966 Wurtz 2925.14 3,351,996 11/1967 Fiore 2925.l5

PAUL M. COHEN, Primary Examiner U.S. Cl. X.R. 

